High amounts of estrogen or prolonged exposure throughout your life may increase your risk of breast cancer. But there are steps you can take to lower your overall risk.

Estrogen itself isn’t always dangerous and is, in fact, a necessary component in human development. However, in some cases, increased estrogen exposure over your lifetime may increase your risk of developing breast cancer.

In this article, we take a closer look at estrogen, including when and why it may increase your risk of developing breast cancer and steps you can take to lower your risk.

What role does estrogen play in the human body?

Estrogen is a type of female reproductive hormone primarily produced by the ovaries, skin, and fat tissues. It’s also responsible for helping the human body develop female sexual characteristics. In particular, estrogen plays a role in:

  • the development of breast, vulva, and vaginal tissues
  • assisting in pregnancy and contraception
  • bone density and development
  • cholesterol regulation
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In some cases, doctors may prescribe estrogen for:

  • pregnancy prevention
  • moderate acne
  • female hypogonadism
  • advanced prostate cancer
  • menopause symptom relief

Estrogen is a physiological necessity for people assigned male or female at birth. However, it may be harmful in large amounts and may even increase the risk of breast cancer development.

Hormone replacement therapy (HRT)

People assigned female at birth who are postmenopausal and take hormone replacement therapy (HRT), also called menopausal hormone therapy, may especially be at risk.

HRT is sometimes prescribed to help alleviate symptoms of menopause, such as hot flashes, changes in mood, and night sweats, especially if these symptoms interfere with your quality of life. However, the pros and cons of HRT must be weighed carefully because of the risk of breast cancer.

According to the National Cancer Institute, estrogen-only HRT may increase your risk of breast cancer, and this risk does not decrease if you stop taking the medication. On the other hand, combination HRT with estrogen-progestin also increases your risk, but this may go down slightly after you stop taking it.

Other estrogen-related factors

While HRT may increase your risk of breast cancer, there are other factors that can increase your estrogen exposure and subsequent cancer risk. These include:

  • taking oral contraceptives (although more studies are needed)
  • diethylstilbestrol use during pregnancy (prevalent between 1940 and 1971)
  • early menstruation
  • late onset of menopause
  • not having given birth
  • first birth at an older age

Aside from estrogen exposure, other factors may increase your lifetime risk of developing breast cancer.

While having any of these risk factors doesn’t mean you’ll automatically develop cancer, you may consider discussing these with a doctor or healthcare professional to help lower your risk overall.

Overweight and obesity

Having overweight or obesity can increase your risk of breast cancer, especially postmenopause. This is because fat tissues store and release estrogen, which can increase overall estrogen levels.

Additionally, having excess body fat can increase insulin levels, which may in turn raise your risk of developing breast cancer.

Lack of physical activity

Regular exercise not only helps you keep a moderate weight, but it may also lower your risk of developing breast cancer. It’s thought that exercise may help influence hormone levels, as well as tame inflammation.

Aim for about 300 minutes (5 hours) of moderate exercise per week, especially if you’re postmenopausal.

Drinking alcohol

When considering breast cancer, the American Cancer Society (ACS) recommends that people assigned female at birth drink no more than one serving of alcohol per day — if any.

The more alcohol you consume, the greater your risk of developing breast cancer. One drink per day is associated with a 7 to 10% increased risk, while 2 to 3 drinks per day can increase your risk by 20%.

Not having children or breastfeeding

While the exact reasons are unclear, people who have children — especially before the age of 35 — are thought to have a lowered breast cancer risk.

Breastfeeding may also slightly decrease your risk because of a smaller number of menstrual cycles over your lifetime, which decreases estrogen exposure.

Other possible risk factors

Additionally, the ACS outlines other possible risk factors of breast cancer that are “unclear,” meaning that more research is needed to determine whether such factors contribute to this particular type of cancer. These include:

  • a high fat diet
  • environmental chemicals such as plastics and pesticides
  • shifts in melatonin and other hormones because of nightshift work
  • secondhand smoke exposure
  • exposure to radiation, such as that of chest X-rays (especially before the age of 20), because it can increase breast cancer risk for women with inherited changes in the BRCA1 and BRCA2 genes

While there’s no single method of cancer prevention, there are steps you can take to lower your risk of developing breast cancer. The ACS recommends that you:

  • achieve and keep a moderate weight
  • limit (or avoid) alcohol
  • get at least 150 to 300 minutes of moderate exercise per week
  • talk with a doctor about genetic counseling if breast cancer runs in your family
  • consider preventive surgeries (such as breast or ovary removal) or estrogen-blocking medications as recommended by a doctor if you’re deemed high risk

ACS screening recommendations

Breast cancer screening recommendations from the ACS for people assigned female at birth are as follows:

  • Those between 40 and 44 have the option to start screening with a mammogram every year.
  • Those 45 to 54 should get mammograms every year.
  • Those 55 and older can switch to a mammogram every other year, or they can choose to continue yearly mammograms. Screening should continue as long as a person is in good health and is expected to live at least 10 more years.

Everyone should understand what to expect when getting a mammogram for breast cancer screening and what the test can and can’t do. Talk with a doctor about screening recommendations that are appropriate for you.

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What type of breast cancer is caused by estrogen?

Estrogen may contribute to hormone-sensitive breast cancers. For this reason, a doctor will take a small tissue sample to determine whether your breast cancer is estrogen receptor-positive. It’s thought that up to 80% of breast cancers in females and 90% in males are estrogen receptor-positive.

If you receive a diagnosis of estrogen receptor-positive breast cancer, you may need a special type of hormone therapy that blocks estrogen. This may come in the form of selective estrogen receptor modulators such as tamoxifen.

How much estrogen increases your risk of breast cancer?

Estrogen may be prescribed at various doses based on its intended use. There’s not an exact number that’s associated with a definitive development of breast cancer. However, prolonged estrogen exposure may increase your risk of developing breast cancer.

Does estrogen make breast cancer grow?

Estrogen is thought to contribute to the development of hormone-sensitive types of breast cancers. If you receive a diagnosis of early stage estrogen receptor-positive breast cancer, this means that you have cancer cells that contain certain proteins that may activate when estrogen binds to them.

Do the benefits of HRT outweigh the risk of breast cancer?

Whether you should take HRT depends on your overall individual risk of developing breast cancer.

According to one 2021 review, people who were considered low risk before starting HRT for menopausal symptoms had less than a 3% chance of developing breast cancer within 5 years.

However, researchers don’t recommend HRT if you’re at a high risk of developing breast cancer. A doctor can help you assess the benefits and risks of taking hormones for menopausal symptom relief.

Learn more about the benefits and risks of HRT.

Estrogen exposure throughout your life may increase your risk of breast cancer, especially in cases where you may have additional exposure through medications or your environment. However, not everyone who’s exposed to excess estrogen will develop breast cancer.

Talk with a doctor about your own breast cancer risk factors, including whether this type of cancer runs in your family. They can suggest ways to minimize the risk of breast cancer development and discuss regular screenings, which can help find cancer early.